Epitaxial deposition processes are increasingly used for high-speed semiconductor devices, both for silicon and compound semiconductor applications. An epitaxial layer is a carefully grown, single crystal silicon film. Epitaxial deposition utilizes a silicon source gas, typically silane or one of the chlorosilane compounds, such as trichlorosilane or dichlorosilane, in a hydrogen atmosphere at high temperature, typically around 800-1100° C., and under a vacuum condition. Epitaxial deposition processes are often doped with small amounts of boron, phosphorus, arsenic, germanium or carbon, as required, for the device being fabricated. Etching gases supplied to a process chamber may include halocompounds such as HCl, HBr, BCl3, Cl2 and Br2, and combinations thereof. Hydrogen chloride (HCl) or another halocompound, such as SF6 or NF3, may be used to clean the chamber between process runs.
In such processes, only a small proportion of the gas supplied to the process chamber is consumed within the chamber, and so a high proportion of the gas supplied to the chamber is exhausted from the chamber, together with solid and gaseous by-products from the process occurring within the chamber. A process tool typically has a plurality of process chambers, each of which may be at respective different stage in a deposition, etching or cleaning process. Therefore, during processing a waste stream formed from a combination of the gases exhausted from the chambers may have various different compositions.
Before the waste stream is vented into the atmosphere, it is treated to remove selected gases and solid particles therefrom. Acid gases such as HF and HCl are commonly removed from a gas stream using a packed tower scrubber, in which the acid gases are taken into solution by a scrubbing liquid flowing through the scrubber. Silane is pyrophoric, and so before the waste stream is conveyed through the scrubber it is common practice for the waste stream to be conveyed through a thermal incinerator to react silane or other pyrophoric gas present within the waste stream with air. Any perfluorocompounds such as NF3 may also be converted into HF within the incinerator.
When silane burns, large amounts of silica (SiO2) particles are generated. Whilst many of these particles may be taken into suspension by the scrubbing liquid within the packed tower scrubber, it has been observed that the capture of relatively smaller particles (for example, having a size less than 1 micron) by the scrubbing liquid is relatively poor. In view of this, it is known, for example from U.S. 2005/0123461, to provide an electrostatic precipitator downstream from the scrubber to remove these smaller particles from the waste stream.
FIG. 10 illustrates a known arrangement of a scrubber 10 and an electrostatic precipitator 12 for removing solid particulates from a gas stream. A first compartment 14 of the scrubber 10 contains a packed tower 16 of packing material irrigated by a scrubbing liquid, usually water, which is received at a water inlet 18 and sprayed on to the packed tower 16. A sieve plate 20 supporting the packed tower 16 drains scrubbing liquid from the packed tower 14 into a second compartment 22 of the scrubber 10. A drain element 24 drains the scrubbing liquid from the second compartment 22 for recirculation back to the water inlet 18.
A gas inlet 26 conveys the gas stream into the second compartment 22 of the scrubber 10. The gas passes upwards through the holes of the sieve plate 20 into the first compartment 14, wherein acidic gases and the larger solid particulates contained in the gas stream are transferred to the scrubbing liquid passing downwards over the packing material. The scrubbed gas leaves the scrubber 10 through gas outlet 28 located at the top of the first compartment 14, and is conveyed by the gas outlet 30 to the gas inlet of the electrostatic precipitator 12.
The electrostatic precipitator 12 contains two electrostatic chambers 34, 36 each connected at a lower end thereof to an intermediate chamber 38 that conveys the gas from the bottom of one electrostatic chamber 34 to the bottom of the other electrostatic chamber 36. Each electrostatic chamber 34, 36 comprises a centrally located, inner electrode 40 and an outer electrode 42 surrounding the inner electrode 40 and which may be provided by an electrically conducting wall of the chamber 34, 36. Each electrostatic chamber 34, 36 also has a water inlet 44 to which a flow of water is supplied to produce a “curtain” of water flowing downwards around the inner surface of the outer electrode 42 and into the intermediate component 38. A drain element 46 drains the water from the intermediate compartment 38 for recirculation back to the water inlets 44.
The gas inlet 30 is arranged to convey gas to the top of electrostatic chamber 34, and a gas outlet 48 is arranged to convey gas out from the top of electrostatic chamber 36.
During use, a high voltage is applied to each of the inner electrodes 40 to produce an electrostatically charged field, or corona, between the inner and outer electrodes 40, 42 of each electrostatic chamber 34, 36. As the gas passes through the corona, any particulates contained in the gas become electrically charged and are drawn towards the outer electrode 42, where the particulates enter the water curtain and are washed into the intermediate chamber 38. The electrostatic chamber 34, in which the gas flows downwards through the chamber 34 with the water curtain, serves to remove solid particulates from the gas, whilst the electrostatic chamber 36, in which the gas flows upwards against the water flow, serves to remove the finer solid particulates and any water droplets from the gas.
Two separate water recirculation systems are required to provide scrubbing liquid, or water, to both the scrubber 10 and the electrostatic precipitator. Each of these recirculation systems may also include a water treatment unit for removing acidic species from the water prior to its re-use, and this can contribute towards relatively high costs associated with operating the gas treatment apparatus.